Measures: Service costs were calculated using recent estimates of the cost per outpatient visit and day of residential care. Average numbers of visits/days per client, with and without mental disorders, were compared across gender and age subgroups.

Findings: Overall, 22% of clients treated in 2010–2011 reported a past-year mental disorder. The costs of services for these clients were estimated to be between $18.5 and $40.3 million (in Canadian dollars), or 20% of the total service costs for the year. The largest difference in average volume of care was seen for residential treatment, where having a mental disorder was associated with seven fewer days of care. On average, during the period studied, among those without mental disorders, men spent 20 more days in residential care than did women.

Conclusions: At the system-level in Ontario, clients with mental disorders did not account for a disproportionate amount of the total costs of care during the year. While these findings may signal a mismatch between the needs of clients with co-occurring disorders and their use of services, it is also possible that such clients receive additional care in other treatment sectors. Further study is warranted to explore potential unmet needs among clients with co-occurring disorders in addiction treatment.